Please improve plugin dependency handling

Description

While testing the latest RC build (RC3), the djvu plugin was missing from the list of installed plugins. This was simply because i missed the required djvu libs. After installing them the plugins are back.

I like to suggest:

the list of plugins should contain a list of all detected plugins

if the installation of a plugin fails, the dialog should list this plugin together with a (hopefully helpful) error message

I think that in your last bullet point above, you mean "if the loading of a plugin fails," rather than the installation of a plugin.
Correct.

I also think your suggested error message is sufficient. We have a detailed readme with installation instructions.

We have to think about the way we will show the error. From my point of view the check has to be done at application start. Let's assume only the djvu plugin is not available. Does it make sense to show an error dialog if i like to open a pdf file?

I think the error info should be always available from the plugin dialog (as i already mention in the ticket itself). Maybe there is a way to figure out, if a plugin is available for the file type but the loading of the plugin failed. In this case i think we have to present a error dialog.

And an error dialog is also a good idea if there is no plugin available/loadable at all.